A. Field of the Invention
This invention relates in general to cooling systems and, in particular, to a capacity control system for regulating chillers.
More specifically, without restriction to the particular use which is shown and described, this invention relates to an evaporator inlet water temperature control system for inverter-driven centrifugal compressor chillers by which evaporator inlet water temperature is controlled by creating the proper evaporator outlet chilled water temperature.
Large capacity air-conditioning systems commonly have employed centrifugal compressors having guide vanes which are adjustable to control the capacity of the compressor for effective and efficient operation. Control of compressor capacity through adjustment of the vanes alone, has proved to be unsatisfactory because of its resultant low efficiency operation of the system. In an effort to overcome such problems, attempts have been made to control not only the position of the pre-rotation vanes, but to regulate the speed of the electric motor driving the compressor for purpose of an energy effective system and to avoid surge.
One highly effective system which relies on vane adjustment and speed control to achieve superior capacity control of a centrifugal compressor chiller, is disclosed in U.S. Pat. No. 4,151,725 to Kountz et al for A Control System for Regulating Large Capacity Rotating Machinery and assigned to the assignee of this invention. In the system disclosed in U.S. Pat. No. 4,151,725, a control system effectively maximizes efficiency without encountering surge problems by monitoring the temperature of the refrigerant in the condenser discharge line, the temperature of the saturated refrigerant leaving the evaporator, the temperature of the chilled water discharged from the evaporator of the chiller, and the inlet guide vane position. Based on the foregoing four parameters and a set point temperature input, the control system described in U.S. Pat. No. 4,151,725 effectively regulates the refrigeration system by regulating the speed of the compressor and adjusting vane position.
The temperature of the chilled water sensed at the outlet from the evaporator in U.S. Pat. No. 4,151,725 is applied to a circuit which also receives a temperature set point signal from a potentiometer or another suitable unit, such as a thermostat in the building space. Thus, a signal is derived representing the difference, if any, between the set point temperature of the thermostat and the like and the instantaneous load. Greater efficiency could be attained in such a refrigeration system, however, if the inlet water being directed to the evaporator on return from the building is constantly monitored and the capacity control system is responsive thereto to maintain a constant or set point temperature at the inlet regardless of the load conditions. Maintenance of the set point inlet temperature of the water to the evaporator in response to actual load conditions would result in significantly improved efficiency and economy of operation since a variable input related directly to the temperature of the water after circulation is applied to the capacity control system.
B. Prior Art Patents
U.S. Pat. No. 4,112,703 to Kountz discloses a refrigeration control system in which a controlled space is maintained at a set point by adjusting the refrigerant temperature at the evaporator outlet in a refrigeration system. In U.S. Pat. No. 3,751,940 to Norbeck, a refrigeration system is disclosed having circuit means for automatically energizing the PRV motor.